Nuclear Medicine
CLINICAL DECISION SUPPORT
Normal and ectopic gastric mucosa can be demonstrated by the active uptake and secretion of Na[99mTc]TcO4. Ectopic gastric mucosa is found in 60-85% of Meckel’s diverticula.
Meckel’s diverticulum is a congenital pouch in the small intestine caused by incomplete closure of the omphalomesenteric duct. This abnormality occurs in the distal part of the small intestine 40-150 cm proximal to the ileocecal valve. Meckel’s diverticulum affects 1-3% of the population and is asymptomatic in 70-80% of cases. Symptoms related to bleeding or ulceration of the diverticulum usually occur before the age of two. Symptoms usually comprise painless gastrointestinal blood loss in children, while adults can present with diverticulitis, invagination, volvulus, or perforation. Bleeding from Meckel’s diverticulum is unusual after the age of forty. Scintigraphy can also demonstrate ectopic gastric mucosa at uncommon locations in children such as in a double loop of the small intestine, in normal small intestine, or in the oesophagus.
Small bowel follow-through examinations using barium sulphate do not rule out Meckel’s diverticulum. Scintigraphy is therefore the examination of choice in the evaluation of Meckel’s diverticulum. Depending on the scintigraphic findings, supplementary examinations may be performed (ultrasound, abdominal CT, barium examinations of the bowel, and contrast angiography) to demonstrate not only the presence of Meckel’s diverticulum but also other pathologies. The sensitivity of this examination is high in young children (approximately 85%) but only 60% in adults. A negative study does not necessarily rule out Meckel’s diverticulum, but simply indicates that no functional ectopic gastric mucosa is present.
For adults, the recommended administered activity is 200 MBq.
In paediatric nuclear medicine, the activities should be modified according to the EANM paediatric dosage card (https://www.eanm.org/publications/dosage-calculator/). The minimum recommended activity to administer is 20 MBq.
The effective dose per administered activity is [3]:
Na[99mTc]TcO4: 13 µSv/MBq
The range of the effective dose for the suggested activity is 2.6 mSv.
Caveat
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."
The flow images demonstrate initial blood flow to the liver, spleen, and the large abdominal and pelvic vessels. Later images demonstrate the concentration of radioactivity in the gastric mucosa, bladder, kidneys, and transport of the radiopharmaceutical to the proximal small intestine.
Functioning ectopic gastric mucosa in Meckel’s diverticula are seen as sharply defined hotspots that appear usually at the same time and with the same intensity as normal gastric mucosa. These abnormalities are often seen in the lower right quadrant of the abdomen adjacent to the navel, but the location can change spontaneously as the patient changes position. Other common locations of ectopic or gastric mucosa are the oesophagus, a double loop in the small intestine or in normal small intestine.
False positive results can be caused by blood pooling in the uterine mucosa, often referred to as uterine blush. During the first half of the examination, there is local uptake of activity above the bladder which fades or disappears during the test. This is caused by mucosal hyperaemia in the second half of the menstrual cycle.
False positive results include the presence of free Na[99mTc]TcO4 in the bowel lumen excreted from the stomach, activity in the renal pelvis, the ureter, or in a bladder diverticulum. The transport of activity to the small intestine will be inhibited if glucagon has been administered.
Several other abdominal abnormalities can cause false positive results. These include bowel inflammation, haemangioma, abscesses, vessel defects, and small bowel tumours (e.g. leiomyosarcoma and carcinoids). In some cases, an additional SPECT/CT could be necessary to distinguish these.
False negative results can be caused by small Meckel’s diverticulum (<1 cm2), Meckel’s diverticulum without gastric mucosa, Meckel’s diverticulum with a faster secretion the radiopharmaceutical than normal gastric mucosa, perchlorate use prior to the test, or when pentagastrin use results in increased wash out. A SPECT/CT could increase sensitivity for detection of smaller lesions. In addition, previous barium studies could hinder the detection of Meckel’s diverticulum, and the Meckel’s diverticulum might not be detected due to over projection of activity in the urinary bladder.
The patient should be NPO. Gastroscopy, colonoscopy, and x-ray examinations of the small or large bowel using contrast media should not be performed within three days prior to scintigraphy. If an examination has been performed using barium sulphate contrast agent, then a plain abdominal x-ray should be taken before performing scintigraphy in order to determine the presence of any remaining barium.
Medication which affects bowel motility should be discontinued three days beforehand.
The sensitivity of this test can be increased by administering cimetidine (Tagamet®) or ranitidine (Zantac®). These H2-receptor antagonists inhibit the secretion of Na[99mTc]TcO4 into the gastric lumen, and thereby suppress abnormal transport distally to the small intestine. Ranitidine is preferable to cimetidine both from a pharmacological and a practical point of view.
In the past, a combination of pentagastrin and glucagon was used to increase the accuracy of the test. Pentagastrin stimulates the uptake of Na[99mTc]TcO4 in the gastric mucosa, and glucagon slows down gastrointestinal peristalsis and thereby preventing a false positive result at the level of the Meckel’s diverticulum. Pentagastrin, however, can give false negative results due to an increased wash-out effect. Besides, it can induce peptic ulcer formation.
The influence of the proton pump inhibitor omeprazol (Losec®) on the test results has not been investigated.
The thyroid must not be blocked using perchlorate, because this could cause false negative results.
Joint EANM-SNMMI Guidelines are available at http://tech.snmjournals.org/content/42/3/163.short